1. Field of the Invention
This invention relates generally to a diode frequency mixer and, more particularly, to a star diode frequency mixer implemented in a monolithic integrated circuit.
2. Discussion of the Related Art
The properties of electrical components such as amplifiers, filters and detectors typically function at different frequencies. Therefore, it is necessary to shift signals from one component to another, or from one circuit to another, to a frequency where the different components can perform their functions most effectively. Frequency mixers which translate a signal from one frequency to another frequency where the signal can be amplified or processed more effectively are known in the art. It is important that a mixer perform the translation of the frequency with minimal added noise or distortion. A frequency mixer will either add or subtract an input signal to a local oscillator (LO) in order to generate a higher or lower intermediate frequency. A signal which is added to a local oscillator is generally referred to as upconverting, and a signal which is subtracted from a local oscillator is generally referred to as downconverting.
Theoretically, any non-linear or rectifying device can be used as a mixer. Practically, however, particularly at frequencies within the microwave range, Schottky-barrier diodes provide the necessary electrical properties desirable for a frequency mixer. Two general types of diode mixers are known in the art. These types of diode mixers have been referred to as a diode ring mixer and a diode star mixer because of the shape of their electrical configuration. Although the diode ring mixer has traditionally been the most common type of mixer due to its simplicity, the diode star mixer offers a number of advantages not found in the ring mixer. These advantages include low parasitic inductance making possible broad intermediate frequency bandwidths, intermediate frequency DC coupling, and good intermediate frequency isolation. Therefore, diode star mixers are becoming increasingly more popular. For a more detailed discussion of the electrical configuration and operation of diode mixers of these types, see S. A. Mass, Microwave Mixers (2nd edition), Artech House, Norwood, Mass. 1992, herein incorporated by reference.
Diode star mixers of the type discussed above use coupled-line baluns. Baluns of this type generally have poor performance unless the coupled lines have high even-mode impedance and closely matched even and odd-mode phase velocities. In order to provide these features, traditional diode star mixers require that the mixer be configured on a suspended, low dielectric substrate. Therefore, at the present time, diode star mixers have only been implemented in either a multilayered microstrip/stripline or a waveguide environment. Such a configuration has been found impossible for a monolithic circuit, thus limiting the use of these types of mixers. Consequently, the prior art star diode mixer has to be designed as a hybrid circuit that generally requires a labor intensive assembly. This not only increases the cost, but provides a parasitic inductance associated with the hybrid circuit that degrades the performance of the mixer.
What is needed is a diode star mixer incorporating coupled-line baluns which have good balance and good port-to-port isolation, but which can readily be configured in a monolithic circuit. It is therefore an object of the present invention to provide such a diode star mixer.